Media on usb hard disk drive not listed

My USB hard disk is recognized automatically. Looked in /media or /mnt/USB via ssh I can see all my files (mp3 and flac, m3u). But if I want to play some files by Music Library/USB/… via the normal front-page only m3u-files are listed, even some folders are missing. Also if I add my hard drive to the sources via \Volumio\usb the size is only 826.39 MB (from about 300 GB)
What can I do to get my files playing?

Volumio Information

Volumio Version: 2.861
Hardware: RPi
DAC: Allo Boss

You don’t need playlist files, Volumio doesn’t use them. So I’d clean those up as a first step.
Make sure all the files are properly tagged with album, artist, and title.
Make sure they are consistently tagged and test the results after you’ve done a couple of artists and/or albums.

It took me a while to get things listing and displaying consistently.